1. Field of the Invention
The present invention relates to crossed-field devices such as magnetrons, and more particularly, to an output system for coupling RF energy out of a magnetron that damps undesired modes of oscillation of the magnetron.
2. Description of Related Art
Magnetrons are a type of crossed-field device that are commonly used to generate high power microwave energy for assorted applications, such as radar. A magnetron typically comprises a cylindrically shaped cathode that extends axially along a central axis of an anode structure comprising a plurality of anode vanes that extend radially from an annular anode ring. A space defined between the cathode surface and the anode structure provides an interaction region, and an electric potential is applied between the cathode and the anode forming a radial electric field in the interaction region. An axial magnetic field is provided in the interaction region in a direction perpendicular to the electric field by polepieces that focus magnetic flux from magnets disposed externally of the interaction region. The cathode may be provided with an internal heater disposed below the surface of the cathode to heat the cathode surface to a temperature sufficient to cause thermionic emission of electrons therefrom. The emitted electrons are caused to orbit around the cathode in the interaction region due to the axial magnetic field, during which they interact with an electromagnetic wave that is caused to move on the anode structure. The orbiting electrons give off energy to the electromagnetic wave, thus resulting in a high-power microwave output signal.
In order to put the high-power microwave output signal to use, an output circuit is provided to couple into the electric or magnetic (or both) fields that are supported in the interaction region in order to couple the output signal out of the magnetron. A typical output circuit includes a wire loop disposed in one of the cavities of the anode defined between adjacent anode vanes. The degree of coupling must be selectable, either at the design stage or as a direct adjustment on a xe2x80x9ccold-testxe2x80x9d as the magnetron is being built, and must remain relatively constant once selected.
A common problem with magnetrons is that they have a tendency to oscillate in a mode known as the xcfx80xe2x88x921 mode instead of the desired mode (called the xcfx80 mode). A known technique for promoting oscillation in the xcfx80 mode is to provide an annular strap that couples alternating ones of the anode vanes. Another technique for promoting the xcfx80 mode is the use of an external resonant cavity of high Q. Other known techniques have focused on suppressing the xcfx80xe2x88x921 mode, such as to orient the fields of the xcfx80xe2x88x921 mode in such a way that neither of its doublets is left lightly coupled or uncoupled to the output system. The output circuit often represents a significant source of damping to the undesired modes oscillating in the RF structure. Should one of the doublets of the xcfx80xe2x88x921 mode be left lightly coupled or totally uncoupled to the output circuit, then it is effectively free of the main source of damping of oscillations within the magnetron. In this situation, the xcfx80xe2x88x921 mode may build in amplitude to such an extent that its field pattern disturbs and eventually dominates the electron trajectories. Such disturbances tend to degrade the stable and effective operation of the magnetron. There are various known techniques to achieve the orientation of the fields of the xcfx80xe2x88x921 mode, e.g., slots in the cavity backs, strap-breaks, etc. Nevertheless, these techniques add complexity and manufacturing cost to the magnetron, and also introduce inductance and capacitance that alters the resonant characteristics of the magnetron.
Accordingly, it would be desirable to provide an output system for a magnetron that maintains coupling to both doublets of the xcfx80xe2x88x921 mode in order to provide effective damping of undesired oscillations in the magnetron. It would also be desirable to provide an output system that can be constructed and optimized separate from the magnetron structure to provide a consistent level of performance among production devices.
In accordance with the teachings of the present invention, an output circuit is provided for magnetron that enables coupling into two adjacent anode cavities, thereby ensuring coupling to the xcfx80xe2x88x921 doublets in at least one of the adjacent anode cavities. As a result, it is unnecessary to implement any method of xcfx80xe2x88x921 mode orientation. Moreover, the two adjacent anode cavities are symmetrically loaded. Therefore, the xcfx80xe2x88x921 mode field pattern is more uniform around the RF structure than with prior art coupling methods that couple to only a single cavity.
More particularly, the magnetron comprises an anode ring concentrically disposed around and spaced from a cathode. The anode ring further comprises a plurality of anode vanes extending radially toward the cathode with cavities being defined between adjacent ones of the plurality of anode vanes. One of the plurality of anode vanes provides an output vane whereby a high power microwave signal is developed in first and second output cavities disposed at either respective side of the output vane. The high power microwave signal is coupled out of both the first and second output cavities by a coaxial transmission line that includes first and second coupling loops disposed in the first and second output cavities, respectively. The output vane further comprises an opening at a central portion thereof. The first and second coupling loops share a common central portion that extends through the opening of the output vane without contacting the output vane. The common central portion extends outwardly of the anode ring to permit communication of the high power microwave signal therefrom. The output circuit further comprises an outer body portion that engages a corresponding bore extending radially through the anode ring. The first and second coupling loops are coupled to an end of the outer body portion that engages the anode ring. The first and second coupling loops are oriented substantially perpendicular to the output vane. The output circuit further comprises an antenna for communication of the high power microwave signal therefrom.
A more complete understanding of the double loop output system for a magnetron will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.